Re: Newbie storage questions... (RAID5, SANs, SCSI)

On 28 Nov 2003 23:04:02 -0800, [EMAIL PROTECTED] (Robert Wessel)
wrote:

>"Nik Simpson" <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
>> IIRC, the original RAID definition for RAID3 is striping at the byte level,
>> not the bit level, perhaps you are thinking of RAID2.

>Both RAID2 and RAID3 are (effectively) striped at the bit level - the
>smallest addressable unit ("sector" or "block") of the array is split
>across all the drives, thus reading (or writing) that unit requires
>hitting all those drives (hopefully in parallel).

This is, perhaps, a little misleading: the definitions of levels 2 and
3 do *not* detail precisely how the data is split, just that it is
split *below* the level of the host addressable block (whatever that
it).  An easy RAID3 implementation might present itself to the host as
a 2K block device (which would confuse most hosts, but from the device
side, we can *pretend* that that is some else's problem... at least,
until we want to actually make money from the device).

In that instance, the data may be striped in any convenient fashion:
bit, byte, word, longword, doubleword, 64 byte, 512 byte -- the latter
being, perhaps, the easiest.

All that matters is, as Robert notes, that a single block host IO
involves all the disks.

>  What's different is
>how the error correction works.  In RAID2 an EC scheme is used on a
>bit-by-bit basis, in RAID3 you've got a block parity scheme just like
>in RAID4/5.  I've never actually seen a RAID2 implementation, but it's
>possible someone has one somewhere.

Someone did have a RAID2 system, using 37 disks: 32 data plus 5 ECC,
and a 16KB "native" block size.  It was one of the HPC manufactures
(Thinking Machines, perhaps).

Malc.